Dr. Adam G. Riess, an astronomer at the Space Telescope Science Institute, and professor in physics and astronomy at The Johns Hopkins University, both in Baltimore, Md., today was awarded the 2011 Nobel Prize in Physics by the Royal Swedish Academy of Sciences. The Academy recognized him for his leadership of the High-z Team's 1998 study that provided the first direct and published evidence that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained "dark energy" filling the universe. Riess will receive a medal and a diploma and will share a $1.49 million cash award to be presented at a ceremony in Stockholm, Sweden, in December.

In 2002, Dr. Riess led the team to find 25 of the most distant supernovae known with the Hubble Space Telescope. This work culminated in the first highly significant detection of the preceding, decelerating epoch of the universe, helped to confirm acceleration, and began characterizing the time-dependent nature of dark energy. NASA identified it as the top achievement of the Hubble Space Telescope to date.

Dr. Riess shares this year's Nobel award with Saul Perlmutter, an astrophysicist at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, whose Supernova Cosmology Project Team published similar results shortly after Riess's and Brian Schmidt's of the Australian National University, a teammate of Riess's. Perlmutter receives one half of this year's prize. The other half of the prize is jointly awarded to Riess and Schmidt.

Although Hubble played a major role in the discovery of dark energy, nearly every major observatory on Earth contributed to the study of this mysterious energy. Ground-based telescopes run by the National Optical Astronomy Observatories, especially the 4-meter Blanco telescope at the Cerro Tololo International Observatory in Chile, and at the Kitt Peak National Observatory in Arizona, as well as European telescopes on the Canary Islands, are credited with discovering the majority of the supernovae ultimately used to track the expansion rate of the universe. The astronomers also used the W. M. Keck Observatory in Hawaii, the MMT Observatory in Arizona, and European Southern Observatory's 3.6-meter telescope in Chile to measure the spectra of the discovered supernovae and the distances of their host galaxies.

Riess has been awarded prizes including the Warner, Shaw, Gruber, and Sackler, is recipient of a MacArthur Fellowship, and was elected to the National Academy of Sciences in 2009.